> ## Documentation Index
> Fetch the complete documentation index at: https://docs.uplift.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Contact Phase X Factor

> Angle between the shoulders and hips in the transverse plane (twisting) at apex (ball contact).

<img src="https://mintcdn.com/upliftlabs/Ku7T6jLWrJyoK7uq/biomechanics/activities/tennis/images/hip-shoulder-separation.png?fit=max&auto=format&n=Ku7T6jLWrJyoK7uq&q=85&s=35a5423171acfdb9babf91f7de24114d" alt="X factor as viewed from above" width="500" height="500" data-path="biomechanics/activities/tennis/images/hip-shoulder-separation.png" />

## Technical Details

* **Variable Type:** Discrete Metric
* **Data Type:** Float
* **Units:** degrees (°)
* **Column Name:** `contact_phase_x_factor`
* **Measurement:** Shoulder-hip separation angle (transverse plane) at [Apex](/biomechanics/activities/tennis/events/apex)

## Description

Contact phase X factor is the twist angular difference between torso and pelvis rotation (transverse plane) at ball contact.
It indicates how much upper-body rotation has been "uncoiled" at contact and can be compared with [Loading Phase Shoulder Hip Separation](/biomechanics/activities/tennis/metrics/loading-phase-shoulder-hip-separation).

X factor is also known as hip shoulder separation.

## Use Cases

* Uncoiling and rotation at contact
* Kinetic chain and technique
* Comparison with loading-phase separation